1. Field of the Invention
Embodiments of the present invention relate to transponder, telemetry/receiver, and sensor apparatus and associated methods. More particularly, embodiments of the invention are directed to self-powered such apparatus having design-in frequency control and associated methods. Most particularly, embodiments of the invention are directed to a radioisotope-powered, piezoelectric-surface acoustic wave transducer and associated methods.
2. Description of the Related Art
Microelectromechanical systems (MEMS) have become pervasive through various applications in everyday life. The small size of MEMS and their fabrication materials naturally offer the opportunity for the integration of these structures with integrated circuits to provide autonomous Microsystems that do not require external power sources. The availability of a small size, reliable, temperature insensitive, and long operating internal power source having a long shelf life can significantly impact the utility of these autonomous microsystems. Even the smallest conventional batteries may be much larger than the MEMS for which they are supplying power, thus limiting the size reduction of devices for certain applications. In addition, conventional batteries have relatively short useful lifetimes, often on the order of days to, at most, several months, and may not operate well or at all at high or low temperatures. There exist applications where it is desirable to have a power source capable of supplying power to a MEMS for many months, years, and even decades. For example, MEMS-based sensors may be utilized to monitor various structural and environmental conditions and transmit this information to a reception location via optical or radio frequency (RF) communications. If such devices could be provided with power sources capable of supplying power for years or decades without replacement, sensor-based devices, for example, could be permanently embedded in buildings, bridges, etc., utilized in outer-space research, and other such applications envisioned by those skilled in the art.
One or more solutions to the need for a longer life power source are disclosed in U.S. Pat. Nos. 6,479,920 and 7,301,254, the subject matter of which are incorporated by reference herein in their entireties to the fullest extent allowed by applicable laws and rules. The aforementioned '920 patent discloses a device wherein the energy carried by particles emitted by radioactive decay in a radioisotope such as Nickel-63 is captured and converted to mechanical potential energy that is stored in an elastically deformable element. Electrical energy is also stored in the capacitor formed between the radioisotope connected electrodes. The release of the mechanical and electrical energy stored in the deformable element can be utilized to activate other mechanical parts directly or can be converted to electrical energy that can be supplied to drive electronic components such as an integrated circuit. Illustratively, the device includes a substrate such as single crystal silicon, glass, etc., with an elongated cantilever beam affixed thereto at one end, and having a free end. A radioactive source is mounted to the substrate under the free end of the beam and an absorber of radioactively emitted particles is mounted to the free end of the beam. A piezoelectric element having output terminals is secured to the top surface of the cantilever so that the piezoelectric plate will flex and deform with the deformable cantilever. The radioisotope source preferably emits electrons. The emitted electrons are absorbed and retained by the absorber, charging the absorber negatively, whereas the source retains a positive charge. As charge builds up on the absorber and the source, the electrostatic force between these elements increases, bending the cantilever beam so that the absorber begins to approach the source. After a specific length of time, the beam will bend sufficiently such that the absorber makes electrical contact with the source, thereby discharging the charge on these elements and releasing the beam, which resiliently returns toward its rest or normal position as it releases the potential energy stored in the bent beam. In doing so, the stress imposed on the piezoelectric plate is released, which generates a pulse of electrical power at the output terminals of the piezoelectric element. The electrical power generated by the piezoelectric element may be connected from its output terminals to a load, such as a radio frequency coil. The capacitance of the piezoelectric transducer element connected to the coil provides a resonant tank circuit that produces an electrical oscillation at a characteristic frequency, which is excited by the pulse of output voltage from the piezoelectric transducer. This voltage may be rectified and stored on a storage capacitor for use by other electronic components, and the high frequency oscillation may also be utilized to provide a radio (RF) signal that can be detected by a remote detector. In addition to the mechanical to electrical conversion, the stored charge in the capacitor is suddenly released creating a current impulse that in turn excites all of the fundamental cavity modes of the container incorporating the cantilever structure. The RF modes excited in the cavity and across the piezoelectric element electrical circuit can be coupled. The RF energy can be radiated away from the device to be picked up by a distant receiver.
A shortcoming of this device is that the frequency of the output RF signal is determined by the equivalent capacitance and inductance of the system. The resonant cavity and the equivalent circuit quality factor is generally low such that the energy in the pulse is distributed over a range of frequencies preventing precise frequency measurement over any appreciable distance in the presence of phase noise in receiving electronics. Furthermore, the cavity and circuit resonance frequency is a function of the dimensions and dielectric properties of the components, which can vary from one device to another, as non-lithographic methods are generally used to fabricate the devices. A tighter tolerance over the transmitted frequency is highly desired for specifying a given frequency to one transmitter, and also being able to make a narrowband pulse measurement that can generally be done with higher signal to noise ratio as the noise is lower in a narrow band of detection.
In view of the shortcomings and challenges associated with the prior art, the inventors have recognized a need for, and benefits of, a self-powered device as described herein above in which the RF frequency can be precisely controlled. The inventors further recognized the advantageous applications of such a device including, for example, as an autonomous transponder, a data telemetry and transmission device such as an RF identification tag (RFID, an autonomous sensor, and others recognized by those skilled in the art.
These and other advantages and benefits are achieved by the embodied invention, which will be described in detail below and with reference to the drawings.